1. Field of the Invention
The invention relates to a protective circuit for the overvoltage protection of a telephone subscriber line interconnect circuit of a digital time multiplex-communication network which includes an electronic interface circuit; and for the overvoltage protection of the electronic switches associated with the subscriber line circuit for test accessibility. Each conductor of a core pair of the connected subscriber line circuit for each corresponding connection of the subscriber line interconnect circuit has provided an isolation of the separate conductors of the core pair from the connected subscriber line circuit, and a separate test access connection to the conductors of the core pair and to the corresponding connections of the active subscriber line interconnect circuits.
2. Description of the Prior Art
The necessity for a protective circuit is that the the interface circuit of the subscriber line interconnect circuit includes electronic circuit elements, as does the test accessibility circuits. Prior art subscriber line interconnect circuits, in which the interface circuit include repeating coils and voltage stabilizing resistors, and in which test accessibility is by means of electromagnetic relays, can withstand overvoltages in the order of 1 kV without damage, so that a protective circuit is not required.
A protective circuit in accordance with the present invention must meet the requirement that the resistance symmetry imposed upon the cable conductors must remain unaffected. Furthermore, in the event of a current interruption in the subscriber line interconnect circuit resulting from a malfunction, it must be acertained whether this interruption is a result of the subscriber line interconnect circuit itself, or of a defect in the protective circuit.